After a query has produced an output table (after the select
list has been processed) it can optionally be sorted. If sorting
is not chosen, the rows will be returned in random order. The
actual order in that case will depend on the scan and join plan
types and the order on disk, but it must not be relied on. A
particular output ordering can only be guaranteed if the sort
step is explicitly chosen.

column1, etc., refer to select
list columns. These can be either the output name of a column
(see Section
2.3.1) or the number of a column. Some examples:

SELECT a, b FROM table1 ORDER BY a;
SELECT a + b AS sum, c FROM table1 ORDER BY sum;
SELECT a, sum(b) FROM table1 GROUP BY a ORDER BY 1;

As an extension to the SQL standard, PostgreSQL also allows ordering by arbitrary
expressions:

SELECT a, b FROM table1 ORDER BY a + b;

References to column names in the FROM clause that are renamed
in the select list are also allowed:

SELECT a AS b FROM table1 ORDER BY a;

But these extensions do not work in queries involving UNION,
INTERSECT, or EXCEPT, and are not portable to other
DBMS.

Each column specification may be followed by an optional
ASC or DESC
to set the sort direction. ASC is
default. Ascending order puts smaller values first, where
"smaller" is defined in terms of the
< operator. Similarly, descending
order is determined with the >
operator.

If more than one sort column is specified, the later entries
are used to sort rows that are equal under the order imposed by
the earlier sort specifications.